Cryptococcosis is a major cause of mortality in patients with AIDS, causing over 600,000 deaths per year worldwide. In recent years, cryptococcosis has also emerged as a major problem among organ transplant recipients. The treatment of cryptococcosis is unsatisfactory since the disease is associated with a high morbidity and mortality even following treatment with antifungal drugs. There is an acute need for new approaches to treatment in these populations at risk. Passive antibody therapy is a particularly promising approach. This competing application proposes a research plan to investigate a new problem in immunology, which has direct relevance to the development of antibody therapy for C. neoformans-related diseases, passive antibody therapies in general and our understanding of humoral immunity. In the past funding period, we have established that the constant region (C) domain of an immunoglobulin can affect the specificity of the antibody by a mechanism that is proposed to involve structural changes in the antigen-binding site. This finding challenges the long-held dogma in immunology that viewed immunoglobulin molecules as bifunctional molecules with two independent domains, composed of a C region and a variable (V) region. Our findings challenge this dogma, and in doing so suggest a need for revisions to our views on idiotype regulation, isotype restriction, generation of antibody responses, and generation of diversity in antibody repertories. At a practical level, these findings are critically important for the design and development of chimeric and humanized antibodies for human therapy and impact our analysis of vaccine responses. The research program proposes four aims: 1) To determine the structures and antigen contact residues for V-region identical Fab fragments derived from IgG1, IgG2a, IgG2b and IgG3; 2) To identify the amino acid that mediate isotype-related differences in specificity among Abs with identical V regions; 3) To establish whether constant region-mediated changes in specificity extend to IgM, IgE and IgA isotypes; and 4) Construction of a second-generation mouse-human chimeric Ab suitable for human trials. At the completion of these studies we will know how the C region affects V region conformation to influence affinity, specificity, and idiotype and we will have improved immunoglobulins to C. neoformans glucuronoxylomannan suitable for subsequent clinical studies.